Blocking outgoiing mails by certain emaiil address

Dear Sirs, 

I need your help very badly. 

We have a Redhat Enterprise mail server using sendmail. How can I restrict 
only few users (most of them our managers) can send/receive email outside of 
our domain. The rest of the users are blocked from sending outside of our 
domain but can send internal mails. How can I do this? 

Your immediate response would be a great help. 

Thank you. 

-Iris
--
Philippine Linux Users' Group (PLUG) Mailing List
plug-sJCYDywXB4m9hMQLrSLElQC/G2K4zDHf@xxxxxxxxxxxxxxxx (#PLUG @ irc.free.net.ph)
Official Website: http://plug.linux.org.ph
Searchable Archives: http://marc.free.net.ph
.
To leave, go to http://lists.q-linux.com/mailman/listinfo/plug
.
Are you a Linux newbie? To join the newbie list, go to
http://lists.q-linux.com/mailman/listinfo/ph-linux-newbie